SACRAMENTO — A three-quarter-mile stretch of coastline near Isthmus Cove on Santa Catalina Island, described by marine biologists as having one of the island's richest concentrations of sea life, will be set aside in January as a marine-life refuge.

The Department of Fish and Game expects to appoint a director of the refuge within the next couple of months to ensure that the sanctuary can open by the first of the year, the earliest date allowed under legislation signed by Gov. George Deukmejian.

Al Petrovich, chief of the department's marine resources division, said that the department will most likely select as director the incoming head of the Catalina Marine Science Center, a 20-year-old research and teaching facility next to the refuge. The center, run by USC, pushed for the refuge designation to protect sea life from divers, fishermen and recreational boaters.

The science center is without a chief, but Ann M. Muscat, who held the post until early this year, said USC expects to replace her before the end of the year. Muscat, science curator at the Museum of Science and Industry, was retained by USC to pursue the legislation.

The measure, written by Sen. Robert G. Beverly (R-Manhattan Beach), creates a refuge from Chalk Cliff to Blue Cavern Point, an area that encompasses Fisherman's Cove and a 100-yard-wide strip of ocean to the east.

The legislation prohibits unauthorized vessels from entering the area and bars divers and fishermen from taking fish or marine plants from it. Because the area offers natural protection from storms, the measure allows boats to enter the refuge during an "emergency caused by hazardous weather."

Bobette V. Nelson, a marine biologist at UC Santa Barbara, said in written comments to the Assembly that the refuge will benefit both researchers and fishermen. Nelson said Fisherman's Cove will provide a breeding area for marine plants and animals that could help restock nearby waters depleted by fishing.

But recreational fishermen, who describe the area as one of the best along the island for fishing, have opposed the designation. About 40 fishing boats based in Redondo Beach, San Pedro and Long Beach regularly bring anglers to the area.

"They say we have all of the island to fish," said William A. Nott, president of the Sportfishing Assn. of California, in an interview in June when an Assembly committee was considering the legislation. "That is not so. . . . That is one of the few areas when you get west of the middle of the island that you can do any good."